An exhaust gas discharged from diesel engines contains particulate matter (PM) comprising as main components carbonaceous soot and soluble organic fractions (SOFs) comprising high-boiling-point hydrocarbon components, which are likely to adversely affect humans and environment when discharged into the air. Accordingly, ceramic honeycomb filters have conventionally been attached to exhaust pipes of diesel engines for removing PM.
One example of ceramic honeycomb filters for capturing PM in the exhaust gas is shown in FIGS. 1 and 2. A ceramic honeycomb filter 10 comprises a ceramic honeycomb structure comprising porous cell walls 2 defining a large number of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 and a peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c sealing the exhaust-gas-inlet-side end surface 8 and exhaust-gas-outlet-side end surface 9 of the flow paths 3 and 4 alternately in a checkerboard pattern. The peripheral wall 1 of the ceramic honeycomb filter is fixed by grip members (not shown) of metal meshes or ceramic mats, etc. to prevent movement during operation, and disposed in a metal container (not shown).
In the ceramic honeycomb filter 10, an exhaust gas is cleaned as follows. As shown by dotted arrows in FIG. 2, an exhaust gas flows into the outlet-side-sealed flow paths 3 opening on the inlet-side end surface 8. While passing through the cell walls 2, particularly through communicating pores on and in the cell walls 2, PM in the exhaust gas is captured. The cleaned exhaust gas flows from the inlet-side-sealed flow paths 4 opening on the exhaust-gas-outlet-side end surface 9 to the air.
PM continuously captured by the cell walls 2 clogs communicating pores on and in the cell walls 2, resulting in increased pressure loss when the exhaust gas passes through the ceramic honeycomb filter. Accordingly, PM should be burned off to regenerate the ceramic honeycomb filter before the pressure loss reaches a predetermined level. The ceramic honeycomb filter should meet the requirements of a high capturing ratio of particulate and low pressure loss. However, because these requirements are in a contradictory relation, the optimization of porosity, pore volume, the size of pores on the cell wall surface, etc. have conventionally been investigated to meet both requirements.
To cope with increasingly stricter recent exhaust gas regulations, investigation has been conducted on exhaust-gas-cleaning apparatuses comprising both of a NOx-removing SCR apparatus and a particulate-removing honeycomb filter, and the honeycomb filter is required to have better pressure loss characteristics than those of conventional ones.
PM includes a large number of so-called nano-particles having diameters of 50 nm or less. When inhaled, these nano-particles are more attached to respiratory organs than larger particles in the same weight. Also, because nano-particles have larger surface areas per volume, toxic chemical substances adsorbed onto their surfaces likely make PM particles more toxic. Because the amount of nano-particles in PM is small in terms of weight, current PM weight regulations do not work well. Future exhaust gas regulations are thus expected to require the reduction of the number of nano-particles dominant in terms of number in particles discharged. Accordingly, honeycomb filters are required to have improved PM-capturing ratios not on a current PM weight basis, but on a basis of the number of PM particles, particularly nano-particles, in addition to excellent pressure loss characteristics.
JP 2005-530616 A discloses a ceramic filter constituted by a cordierite honeycomb structure with ends plugged for capturing and burning particulate discharged from diesel engines, d50/(d50+d90) determined from a pore diameter distribution being less than 0.70, a permeability factor Sf when soot is accumulated, which is defined by the formula of [d50/(d50+d90)]/[porosity (%)/100], being less than 1.55, and a thermal expansion coefficient (25° C. to 800° C.) being 17×10−7/° C. or less. JP 2005-530616 A describes that with such a pore structure (pore size distribution and the communications of pores), small pressure loss can be kept even when carbon soot is accumulated.
JP 2002-219319 A discloses a porous honeycomb filter formed by a material whose main crystal phase is cordierite having a controlled pore size distribution, the pore size distribution being such that the volume of pores having diameters of less than 10 μm is 15% or less of the total pore volume, the volume of pores having diameters of 10-50 μm is 75% or more of the total pore volume, and the volume of pores having diameters exceeding 50 μm is 10% or less of the total pore volume. JP 2002-219319 A describes that because of the above pore size distribution, this porous honeycomb filter has high efficiency of capturing PM, etc., with suppressed pressure loss increase due to the clogging of pores. JP 2002-219319 A also describes that such pore size distribution can be controlled by adjusting the particle size of a silica component, one of cordierite-forming materials, and by lowering the concentration of kaolin.
JP 61-129015 A discloses an exhaust-gas-cleaning filter having small pores having diameters of 5-40 μm and large pores having diameters of 40-100 μm on at least inlet path surfaces of cell walls, the number of small pores being 5-40 times that of large pores, pores on the surface communicating with pores inside the cell walls. JP 61-129015 A describes that this exhaust-gas-cleaning filter always exhibits high, substantially constant efficiency of capturing particulate.
JP 2003-40687 A discloses a ceramic honeycomb structure composed of cordierite as a main component, and having porosity of 55-65% and an average pore diameter of 15-30 μm, the total area of pores opening on the cell wall surface being 35% or more of the total cell wall surface area. JP 2003-40687 A describes that this honeycomb ceramic structure exhibits high capturing efficiency with low pressure loss.
JP 2002-355511 A discloses an exhaust-gas-cleaning filter comprising a ceramic honeycomb structure having a catalyst carried on the cell wall surface, the cell walls having porosity of 55-80%, and the total area of pores opening on the cell wall surface being 20% or more of the total cell wall surface area. JP 2002-355511 A describes that with increased contact area between the catalyst carried on the cell walls and the accumulated PM, this exhaust-gas-cleaning filter exhibits high performance of oxidizing PM by the catalyst with suppressed pressure loss increase.
JP 2002-349234 A discloses an exhaust-gas-cleaning filter having a catalyst carried, the total area of pores opening on the cell wall surface being 30% or more of the total cell wall surface area, the total opening area of large pores having opening diameters of 30 μm or more being 50% or more of the total opening pore area. JP 2002-349234 A describes that such structure provides drastically improved burning efficiency of PM, while preventing damage due to thermal stress.
JP 2003-193820 A discloses a ceramic honeycomb filter having porosity of 60% or more and an average pore diameter of 15 μm or more, the maximum value of an inclination Sn of a curve of a cumulative pore volume relative to a pore diameter on cell walls at the n-th measurement point being 0.7 or more, Sn being expressed by the formula of Sn=−(Vn−Vn-1)/(log Dn−log Dn-1), wherein Dn is a pore diameter (μm) at the n-th measurement point, log Dn-1 is a pore diameter (μm) at the (n−1)-th measurement point, Vn is a cumulative pore volume (cm3/g) at the n-th measurement point, and Vn-1 is a cumulative pore volume (cm3/g) at the (n−1)-th measurement point. JP 2003-193820 A describes that this ceramic honeycomb filter has high resistance to heat stress and heat shock stress, despite high porosity and a large average pore diameter described above.
However, the exhaust-gas-cleaning filters described in JP 2005-530616 A, JP 2002-219319 A, JP 61-129015 A, JP 2003-40687 A, JP 2002-355511 A, JP 2002-349234 A, and JP 2003-193820 A have PM-capturing performance, which increases as PM is accumulated to some extent, but is not necessarily sufficient in an initial use stage before PM is accumulated (when a ceramic honeycomb filter is used from an unused state, or reused after regeneration). Particularly they are insufficient in the efficiency of capturing nano-sized PM, which is important under increasingly stricter exhaust gas regulations. Insufficient capturing efficiency permits harmful nano-sized PM to flow out without captured.
JP 2004-360654 A discloses a ceramic honeycomb filter whose cell walls have porosity of 55-75% and an average pore diameter of 15-40 μm, the total area of pores opening on the cell wall surface being 10-30% of the total cell wall surface area, and pores having equivalent circle diameters of 5-20 μm being 300/mm2 or more among those opening on the cell wall surface. The ceramic honeycomb filter described in JP 2004-360654 A has a PM-capturing ratio improved to some extent in terms of weight, but it is difficult to effectively capture nano-particles in an initial use stage before PM is accumulated. Namely, it has low PM-capturing efficiency in terms of particle number, with low possibility of meeting regulations in terms of number.